Highly universal aliphatic polyesters are attracting attention as a biodegradable plastic, and aliphatic polyesters such as polylactic acid (PLA), polybutylene succinate (PBS), polyethylene succinate (PES) and polycaprolactone (PCL) are available commercially.
Application fields of these biodegradable aliphatic polyesters include film fields such as packaging, agricultural and food films, and films used in these fields are required to have basic performance in terms of high strength, heat resistance, gas barrier properties and biodegradability corresponding to the application thereof.
Among these aliphatic polyesters, PLA is comprised of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA), and since the melting point of single crystals thereof (a crystals) is about 170° C., there are cases in which PLA has inadequate heat resistance as compared with, for example, polyethylene terephthalate, thus requiring improvement thereof.
On the other hand, numerous methods have been proposed for further improving the heat resistance of PLA by blending poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) followed by forming a stereocomplex thereof (for example, Patent Document 3, Patent Document 4, Non-Patent Document 1).
This stereocomplex (SC) is a eutectic mixture of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA), and since the melting point of crystals thereof is about 50° C. higher than α crystals, there are considerable expectations for its use.
However, a stereocomplex is not easily formed simply by melting and mixing PLLA and PDLA and molding the resulting composition into a film, and although the formed film has improved heat resistance, it is difficult to use as a packaging film.
Therefore, the inventors proposed that PLLA and PDLA be melted and mixed, and that the resulting composition be stretched in at least the axial direction under specific conditions to obtain a stretched film having superior heat resistance and toughness (Japanese Patent Application No. 2004-146239).
This stretched film has a diffraction peak (20) as determined by wide-angle X-ray diffraction in the vicinity of 16° (the peak detected in this region may also be referred to as (PPL)), and the total area (SSC) of diffraction peaks (2θ) in the vicinity of 12° C., in the vicinity of 21° and in the vicinity of 24° (peaks detected in this region may be collectively referred to as (PSC)) is less than 10% of the total amount of the area (SPL) of a diffraction peak (PPL) in the vicinity of 16° and (SSC).
Consequently, SC crystals in stretched films are rare as compared with crystals of PLLA and PDLA alone.
Moreover, the present inventors proposed a method for producing a stretched film comprised mainly of SC crystals in which the main diffraction peaks (2θ) as determined by wide-angle X-ray diffraction are in the vicinity of 12°, in the vicinity of 21° and in the vicinity of 24° by carrying out a specific heat treatment on the stretched film (Japanese Patent Application No. 2004-146240).
In addition, a method for improving the gas barrier properties of a polylactic acid-based biaxially stretched film has been proposed comprising the providing of an inorganic oxide, inorganic nitride or inorganic oxide-nitride (Patent Document 5).    [Patent Document 1] Japanese Patent Application Laid-open No. H7-207041    [Patent Document 2] Japanese Patent Application Laid-open No. H8-198955    [Patent Document 3] Japanese Patent Application Laid-open No. H9-25400    [Patent Document 4] Japanese Patent Application Laid-open No. 2000-17164    [Patent Document 5] Japanese Patent Application Laid-open No. H10-24518    [Non-Patent Document 1] Macromolecules, 20, 904 (1987)